Contact printing apparatus with electrostatic hold-down

ABSTRACT

Contact printing apparatus for electrostatic pinning of sheets to each other and in abutment against a transparent plate of copyboard preparatory to exposing the electrostatically laminated sheets to light. The system includes a lid having a conductive pad of resilient, compressively-yielding material which is adapted to be pressed into contact with the laminate and grounded while a D.C. high voltage is applied to a conductive coating on the undersurface of the transparent plate. The built-in grounding of the conductive pad coupled with its resilience greatly increases the electrostatic pulling power by serving as a ground reference.

United States Patent [1 1 Arden et al.

[ CONTACT PRINTING APPARATUS WITH ELECTROSTATIC HOLD-DOWN [75]Inventors: Richard ,I. Arden, Wilmington. DeL;

Harold A. Schweriner, Hatfield, Pa.

[73] Assignee: The Simco Company. Inc., Lansdale,

[22 Filed: Feb. 4, i974 [2H Appl. No.: 439,036

[52] U.S. Cl. 355/78; 355/l l3; 355/l33; 317/262 E [5 l] Int. Cl. G03b27/02 [58] Field of Search 355/78, 113, 133, 3 R; 3l7/262 E [56]References Cited UNlTED STATES PATENTS 2.254.717 9/l94l Weiss 355/l l3 X3,l 74,748 3/1965 Roberts et al. 355/3 X 3.3l8,2l3 5/1967 Kowalski355/l4 1 June 10, 1975 3,359,469 12/1967 Levy et al. 317/262 E 3,480.364l l/l969 Barnett v. 355/75 3,677,640 7/1972 Nakatani et al. v t 355/99 X3,723,00l 3/[973 Zeuncn et al 355/99 X Primary Examiner-Richard L. MosesAttorney, Agent, or Firm-Stanley Bilker [57] ABSTRACT Contact printingapparatus for electrostatic pinning of sheets to each other and inabutment against a transparent plate of copyboard preparatory toexposing the electrostatically laminated sheets to light. The systemincludes a lid having a conductive pad of resilient,compressively-yielding material which is adapted to be pressed intocontact with the laminate and grounded while a DC. high voltageisapplied to a conductive coating on the undersurface of the transparentplate. The built-in grounding of the conductive pad coupled with itsresilience greatly increases the electrostatic pulling power by servingas a ground reference.

2 Claims, 1 Drawing Figure CONTACT PRINTING APPARATUS WITH ELECTROSTATICHOLD-DOWN This invention relates to a contact printing apparatus andmore particularly relates to an electrostatic copyboard in which a sheetof photosensitive paper or film in contact with a negative iselectrostatically laminated to the surface of a transparent platepreparatory to exposure to light transmitted thereupon from the oppositeside of the transparent plate.

In contact printing, one or more sheets of photosensitive material aresuccessively exposed to light through a negative or several negatives inabutment with the photosensitive sheet. During the contact printingoperation, it is essential that the photosensitive sheet and thenegatives be in intimate face-to-face contact with each other while atthe same time allowing the various sheets to be manipulated with respectto each other in order to enable certain indicia thereon to register.

In the past, electrostatic hold-down and clamping systems have beenutilized as a convenient means for pinning the sheets together and inabutment with an underlying surface. However, one of the problemsencountered in electrostatically pinning film, especially heavy gaugestock, to another sheet has been a tendency for one or both members ofthe laminate to curl and produce wrinkles, such conditions causing poorregistration and striations in the printed component.

The present contact printing apparatus employs a transparent plate, onesurface of which has a transparent conductive electrode having a DC.high voltage applied thereto. The photosensitive film and negatives arelaid in face-to-face abutment with each other in contact with thenon-coated surface of the transparent plate, the outermost surface ofthe laminate being contacted by ground to provide a conductive path tothe low side of the DC high voltage. In this manner, opposite chargesare produced at the opposing surfaces of each laminate interface and atthe interface with the noncoated surface of the transparent plate, so asto cause the surfaces to be electrostatically adhered to each other.Light rays are then directed through the transparent plate to expose thephotosensitive portion of the sheet which is not masked by the opaqueportion of the negatives. ln former systems, a ground applicator isrubbed or rolled over the outer surface of the laminate, providing theground contact necessary to produce the electrostatic charges at thevarious interfaces. The contact is momentary and at different areas atdifferent times.

In the present invention, a conductive pad of compressively yieldingmaterial is mounted upon the undersurface of a cover which is presseddown upon the transparent plate and on the laminate pinned thereto. Theconductive pad is connected to ground, and it remains in position duringthe course of the operation. Thus the pad serves as a constant andcontinuous ground reference to enhance the pulling power of theelectrostatic pinning system and to permit replacement of charges lostby leakage from the system. The pressure of the pad, moreover, isuniform and continuous over the entire area of the laminate, bringingthe charges at the various interfaces into the proximity which isrequired for the electrostatic pinning forces to develop maximally overthe entire area. The fact that the pad is grounded also causes theelectrostatic forces to draw the resilient pad downward in still tightercontact. The pressure, the grounding and the resilience of the pad andthe electrostatic charges are all interrelated and work together tosynergistically reinforce the electrostatic attractive forces and tomaintain them at maximum levels, so as to assure intimate contactbetween sheets of the laminate and between laminate and transparentplate without curls and wrinkles.

It is therefore an object of this invention to provide a contactprinting apparatus in which sheets of material, such as photosensitivefilm and negatives, can he electrostatically pinned to each other inintimate faceto-face contact with a transparent plate without curls orwrinkles preparatory to being exposed to light directed through thetransparent plate.

Another object of this invention is to provide a contact printingapparatus in which electrostatic adhesion of the pinned together membersis assured even after repeated manipulations.

Still another object of this invention is to provide a contact printingapparatus in which electrostatic pinning forces are reinforcedsynergistically by physical compressive forces.

Still another object of this invention is to provide a contact printingapparatus in which electrostatic pinning forces are maintained atmaximum level.

Other objects of this invention are to provide an improved device of thecharacter described which is easily and economically produced, sturdy inconstruction and highly efficient and effective in operation.

With the above and related objects in view, this in vention consists ofthe details of construction and combination of parts as will be morefully understood from the following detailed description when read inconjunction with the accompanying drawing in which:

The FlGURE is a perspective view, and partly broken away in section, ofan electrostatic contact printing apparatus embodying this invention.

Referring now in greater detail to the drawing in which similarreference characters refer to similar parts, the contact printingapparatus of the instant invention comprises a base or plaque portion,generally designated as A, and a cover B movable with respect theretofor electrostatically pinning therebetween a photosensitive sheet andmask laminate preparatory to exposure to light.

The base or plaque portion A includes a transparent plate 12, of glassfor example, which is supported within a frame or cabinet 14.Substantially the entire lower surface of the transparent plate 12 iscoated with an electrically conductive material, such as an evaporativefilm of gold, silver, copper or the like, to define a transparentelectrode 16 whose thickness is in the range of a wave length of light.The transparent electrode 16 is connected by way of a lead 18 and aswitch 20 to a high voltage D.C. generator C which is capable ofapplying from 2,500 to 20,000 volts D.C. (positive or negative) to theundersurface of the transparent plate 12. A series of lamps 22 aremounted on a shelf 24 within the cabinet 14 and when coupled to anelectrical power source by way of a switch or timer 26 on the front ofthe cabinet direct rays of light through the coating 16 and thetransparent plate 12 to permit contact printing of photosensitive film28 in abutment with a negative or mask 30, both of which areelectrostatically pinned in face-to-face contact with each and to thenon-coated upper face of the plate 12.

The cover B includes a lid portion 32 made ofa rigid material such aswood. metal or plastic, to the under surface of which is secured aresilient pad 34 having electroconductive characteristics. The pad 34 ispreferably an elastomeric sponge material which is impregnated with aconductive filler, such as carbon or fine metallic powder. The lid 32 ispivotally secured to the back of the cabinet 14 by hinges 3S andbalanced by springs (not shown) to enable movement toward and away fromthe transparent plate 12. In this manner. the cover B may be presseddown upon the laminated sheets 28 and 30 against the plaque portion Aand locked down in light tight disposition thereover by means of latch36. The conductive pad 34 is connected by an internal lead 38 to groundwhereby a conductive path is established back to the low side of thehigh voltage generator C. When the cover B is pressed down and lockedupon the plaque portion A. the resilient pad 34 presses the laminatedfilm 28 and negative 30 into inti mate physical contact with thetransparent plate 12 so as to distribute the physical pressure uniformlythereupon while the electrostatic charges transported via the conductivepad 34 synergistically reinforces such physical pressure.

In order to electrostatically pin sheets to the surface of copyboard.the DC. high voltage from the generator C is applied to the transparentelectrode 16 by closing switch 20. Negative 30 is laid upon the uppersurface of plate 12 and a grounding applicator or roller (not shown) isbrushed over the top surface of the negative 30. By virtue of theconductive path for electrostatic charges being established between theupper surface of negative 30 and the low side of the high voltagegenerator C. charges of opposite polarity will be developed on theopposing surfaces of the negative 30 and the transparent plate 12 so asto effect electrostatic adhesion of the negative to the plaque face. Thefilm 28 is then laid upon the negative 30 which is pinnedelectrostatically to the upper surface of the transparent plate 12 andslidably manipulated on the negative until properly orientecl. Whenregistration of the film 28 on negative 30 has been effected, thegrounding applicator is now brushed over the upper surface of the filmsheet 28 to again define a conductive path for electrostatic chargesfrom the film to the ground side of the high voltage power supply C soas to cause charges of opposite polarity to be constituted on theopposed adjacent faces of the film 28 and the negative 30. The film 28and the underlying negative 30 will now be tightly pinned to gether andto the transparent plate copyboard.

Finally, the cover B is closed down over sheets pinned to the plaque andthe latch 36 engaged. The resilience of the conductive pad 34 by way ofshear action presses out any wrinkles at the laminate interfaces so asto secure efficient distribution of electrostatic charges at suchinterfaces and by way of compressive action brings the charges at suchinterfaces into intimate proximity so as to optimize electrostaticadhesion.

in turn. the attractive forces of the electrical charges pull downwardon the pad 34 reinforcing because of the resilience of the pad. thephysical pressure it can exert. Additionally. the conductive pad 34allows continuous replenishment of charges lost by leakage from thesystem,

While only a single negative 30 is shown in the drawing. it is to benoted that a plurality of overlying negatives may be employed as when acomposite pattern is to be embodied on the film 28. In the latter case.each negative is pinned to the underlying negative in a manner identicalto that described above in relation to the film 28.

Although this invention has been described in considerable detail, suc-hdescription is intended as being illustrative rather than limiting,since the invention may be variously embodied without departing from thespirit thereof, and the scope of the invention is to be determined asclaimed.

What is claimed is:

l. Contact printing apparatus comprising:

a transparent plate of insulating material having first and secondsurfaces and including a transparent electrode of electricallyconductive material in contact with and substantially covering the firstsurface thereof.

means for applying a DC. high voltage with respect to ground on saidtransparent electrode.

means for directing light through said transparent electrode and saidtransparent plate.

a rigid cover for movement toward the second surface of said transparentplate.

a conductive pad of compressively yielding material on said cover forresilient surface-to-surface contact with abutting sheets of materialelectrostatically laminated to the second surface of said transparentplate. and

means for connecting said pad to ground. whereby pressing saidconductive pad against the sheets electrostatically laminated to thetransparent plate will increase the electrostatic force holding thesheets together and to the surface of said transparent plate and providewrinkle-free flat disposition preparatory to exposure of the laminate tolight.

2. The apparatus of claim 1 wherein said conductive pad comprises asponge-like elastomer.

1. Contact printing apparatus comprising: a transparent plate ofinsulating material having first and second surfaces and including atransparent electrode of electrically conductive material in contactwith and substantially covering the first surface thereof, means forapplying a D.C. high voltage with respect to ground on said transparentelectrode, means for directing light through said transparent electrodeand said transparent plate, a rigid cover for movement toward the secondsurface of said transparent plate, a conductive pad of compressivelyyielding material on said cover for resilient surface-to-surface contactwith abutting sheets of material electrostatically laminated to thesecond surface of said transparent plate, and means for connecting saidpad to ground, whereby pressing said conductive pad against the sheetselectrostatically laminated to the transparent plate will increase theelectrostatic force holding the sheets together and to the surface ofsaid transparent plate and provide wrinkle-free flat dispositionpreparatory to exposure of the laminate to light.
 2. The apparatus ofclaim 1 wherein said conductive paid comprises a sponge-like elastomer.